Signal detection and automatic gain control circuit



Dec. 28, 1954 c. w. HoYT A 2,598,357

SIGNAL DETECTION AND AUTOMATIC GAIN CONTROL CIRCUIT Filed March l, 1950 all nventor Gttomeg United States Patent SIGNAL DETECTION AND AUTOMATIC GAN CONTROL CIRCUIT Clyde W. Hoyt, Pennsauken Township, Merchantville,

N. J., assignor to Radio Corporation of America, a corporation of Delaware v Appinaiion March 1, 195o, serial No. 141,036

s claims. (ci. irs-7.3)

The present invention relates to signal detecting systems, and more particularly, although not necessarily exclusively, to combination type signal detecting systems wherein more than one form of signal detection can be derived from a single signal amplifier.

Even more directly, the present invention is involved with an improved signal detecting arrangement which is differential in its loading action. That is to say, when used in connection with AM modulated radio carriers, the present invention allows demodulation and peak detection of the same signal from the output of a single amplifier. The present invention permits the effective gain of a single amplifier to appear greater for the peak detecting action than the gain which appears for the demodulating action.

Because of this latter effect, the present invention is very useful in television receivers having automatic gain control systems. The peak detection of the television signal may be used to develop an automatic gain control potential. On the other hand, the demodulation action of the present invention may be used to demodulate the video signal from the television carrier. rIlie differential loading provided by the present invention allows the AGC potential developed to reflect a higher effective AGC gain than is conventionally obtained with a similar number of circuit elements.

It is well known in television practice that the cornposite television signal used for modulating a television transmitter is complex. It has several parts. The video signal part of the composite signal represents rather low amplitude modulation of the television transmitter. On the other hand, the synchronizing pulse portion of the television signal represents rather high amplitude modulation of the television transmitter.

In television receiver design an effort is made to amplify the modulated radio signal as much as possible before it is demodulated. Practical considerations limit the amount of amplification possible. For instance, cost of the amplifier itself becomes excessive when too much gain is attempted. One of the reasons for this cost is, of course, the relatively wide band width required by television signals. Some television receivers employ an automatic gain control feature. That is, the gain of a signal amplifier is automatically varied in accordance with received signal strength. This gain control is carried on so as to make die amplitude of the signal appearing at the output of the amplier almost constant regardless of control operation considerable amplification of any variation in the signal must be accomplished. The amount of amplification required is usually greater than that normally given to the incoming signal before demodulation. Thus, in the past, it has been common practice to provide special automatic gain control amplifiers in television receivers. These special amplifiers make up for the lack of gain provided by the regular signal amplifiers. Generally, these special AGC amplifiers are required to 'handle only D. C., or at most, narrow band width pulse information.

It is, therefore, an object of the present invention to provide a. signal detecting arrangement which will establish higher effective preampliiication gains for some portions of the detected signal than Vfor other portions of the detected signal. Y I

'It is vanother object of the present invention to provide an improved signal detector arrangement in television receivers utilizing automatic gain control circuits.

In the realization of the above objects and features of advantage, the invention may take many forms. In its particular application to television receivers, the present invention uses a tapped impedance element which may take the form of inductance. The inductance is tapped at a high impedance level and a low impedance level. Signal energy in the television receiver before demodulation is applied to the inductance. A first diode and -a connected load circuit is placed across the low impedance taps of the inductance. This provides a demodulator for the video signal part of the television signal. An automatic gain control circuit is then placed across the high impedance taps of the inductance. This automatic gain control circuit may be nothing more than a second diode with a connected load circuit. Bias means are provided in series with the second diode so as to keep it from conducting except on peaks of television signal synchronizing pulses. The voltage developed across the second diode load then Y'is a measure of the received signal intensity. Therefore,

this voltage may be used as an automatic gain control potential for the television receiver. The higher impedance signal source for the AGC detector gives an eiective boost to the overall AGC gain. This is done without effecting the proper impedance match of the signal demodulator diode to the amplifier feeding the tapped inductance.

A better understanding of the present invention as well as other objects and features of advantage will become apparent from a reading of the following description, especially when taken in connection with the accompanying drawing, which drawing illustrates by a combination block diagram and schematic representation, one embodiment of the present invention as applied to a conventional television receiver.

Turning now to the figure, there is indicated at 10 a television receive'i having an RF tuner, sound channel and video IF amplifier. Details of circuits for such pur- `poses are shown in an article entitled Television Receivers by Anthony Wright appearing in the RCA Review for March 1947. Signals picked up by the antenna 12 are amplied by the RF tuner and converted to an IF frequency. This IF frequency is amplified by those from the previous amplifiers in block 10 are applied to the grid 16 of the tube 14. Anode potential for the tube 14 is derived through a resistor 18 from B-lsource 20. An automatic gain control terminal 22 is also indicated. This terminal is intended to receive a control voltage which will vary the amplification of the gain of the television receiver 10. If this gain is varied in the proper manner, the Voltage applied to the grid 16 of tube 14 will be almost constant over a wide range of signal strengths reaching antenna 12.

According to the present invention, the IF signal apl pearing at the upper terminal of loadresistance 18 is coupled to `a tapped inductance 24. This is -done by' means of capacitor 26. The left-hand diode 28 of the double diode envelope 30 is then usedas a video signal demodulator. 28 in series with another inductance 32, resistance 34 and still another inductance 36.

thought of as a demodulator combination. It is well known that care must be taken to provide the proper de `video IF amplifier may be impaired.

The video signal demodulated by the diode 28 is applied, of course, to a video amplifier 42 which, in turn, drives the grid of an image reproducing tube 44. The demodulated signal appearing at the output of the video amplifier 0 ..42 may be also usedto `synchronize the deflection circuits '46. Details of suitable-circuits for use in blocks 42 and This is accomplished by putting the diode Y These elements may be v terminal; 46 thereof will be much greater than .that avail:4 5

able at the terininal40.r Thus, at rst. blush, it .might appear thatjullv advantage wasvnot being takenof the output of theamplitier 14. But,'a s mentioned before, an attempt tonse this. full voltage would only result in overloadingvthe tube..14. This would not onlyimpairthe'l() characteristics of the video IF amplier,- but would, in fact,.actuallyfreduce the voltage presently lavailable at terminal 46..; Thus, from. the video signal standpoint thegain ofihetelevision lreceivenis pretty lwell-defined at its usechosen to resonate with the associated circuit capacity at some..par.ticular. frequency. accordance with the demands of the particular video I F ampliler. In staggered tuned 1F ampliiiers, the inductance 24 would resonate with the stray circuit capacity, taken in combination wihcap acitors 26 and 2S, at some frequency different from lthe 4other resonant frequencies of previous IF-.ampliiersi Having thus described my invention, what l claim is:

1. In a radio receiver adapted to receive and demodula'teiaradio. carrier-modulated with an intelligence signal.

anda pulse :componenh'said pulse component corresponding to. maximum excursions of the radio carrier and said intelligence signal corresponding to carrier excursions of able maximum. Unless additional amplier tubes i wei'efl less amplitude andhas'said pulse-component, in'combinaused; noadditional. gain up zto the gridof the video ampliier.42 can usually be realized.

However, accordingto the present invention, the higherl signal4 voltage across. inductance 24 is used to good advantage. coupled through .capacitor -48 to the .right-hand diode 50 of the-envelope30.' This'diode is connected in-serieswith a load circuit made Yup of resistor SZin shunt with capacitor .54a A positive bias is placed in-seres with the diode 50.-by;the;bleeder.56. The bleeder 56 -causeszagnet posi- 25 said ii'rstdetecting. circuit vcombination between the low tive -voltage to be .developed across-resistor-SZ which, of course, biases the diode 50 inthe-non-conducting direction. The yrest ofthe load circuitfor the diode 59-is1madegupof-.resistor 58,'v resistor 60 and capacitor 62. This'bias is adjusted so that thediode SGV-will only conduct .on-peaks of sync. Forexample, the waveform 64 shows the .video IF signal across the inductance 24. The video signal con ponentv is sh'ownV at 66 While the .synchronizing component isnshown at 68.V By. biasingoi` .the diode50, .it will be scemi-possible to. allowv conductiontherethrougli onlyby'Y thefpeaks of sync.68.V Y

With the above connections andadjustments,za voltage will berdeveloped across resistor 60 which will be negative withrrespecto ground'. The'magnitude of. the .voltage 40 demodulatedibyisaid second 'detecting circuit combination.

2L1Appar'atus-.according to claim l where said signalv across resistor 60. will be directly proportional .with the amplitude.oflhefreceived signal as measured by the positive .peaksofpsyncz The-potential thus developed 'may bethought of asan'AGC potential and conducted through' tion, a signal ampliier connected for .amplifying a version of said received signal'prior to demodulation, said amplifier employing an output electron discharge tube having an anodeand -acathode between which is connected an The signal. voltage appearing thereacross is 29 output load circuit, an inductance having low impedance taps-and highimpedance taps, means for coupling energy from .saidA amplifier-output circuit-to said'inductance, a.

rst diode connected. in series with a rst load circuitto formadetecting circuit combination, connections placing impedance-taps of'said inductance such to demodulate intelligence signal` acrosssaidlrst load circuit, a second diode connected in'serieswith a second load circuit to forml asecond. detectingv circuit. combination, connections placing saidjsecond detecting circuit combination between thehighfimpedance'taps-on said inductance, biasing means connected in; series with said second detecting circuit combination of such polarity to prevent conduction of said I second diode: forfall'carrier excursions below the amplitude'oflsaid'pulsefcomponent so as to demodulate only high' .amplitude carrier excursions, and means for controllingthezvalue of. lowamplitude signal applied to said iirst detecting'circuit combination by the high amplitude signal amplierns'ofthe staggered tuned variety employing a plurality of vtuned-circuitsand wherein said tapped inductancei: is resonatedby electricallcapacity to form one of resistor-.70 andfbypassed' byy capacitor'72for application 45 kthe tuned circuits of said staggered tuned ampliler.

tostli'e AGCterrninal of the receiver 10.

This .general peak detecting form of developing anV AGC potential; is, 'offcourse, old` in the art. However, the particular-J advantages of the circuit arrangementjof 'the Y present Ainvention gives a considerable boost tothe eiective v50v of said'pulse :component exceeding the maximum excurgain-#.ofthefAGC system. This,.of course, follows inthe light-of. the previous description in Whichffitwas noted thatthezvoltageacross the'total inductance24-fwas much greaterf'thanthe voltage appearing across the tap `4010 ground.. Y

Thepresent inventiony takesIadvantage-of the-factv that theiAGCrectifying or -peak detecting system requires-very littl`e;.power, once the capacitor 62has built'up a charge.- substantially equalto theV peak. of 'sync 682i' Thus, theV sions of said'isignal intelligencegin combination, means foi" controlling theVY gain of.` saidI signal channel in accordancel with a control voltage, a signal'amplier for communicating' signal 'intelligence when demodulated from said lradio least aE grid input circuit anda plate output circuitgcoupling-fr'om the outputof ysaid signal channel to said grid inputv circuit,` aninductanceY element having low impedance -taps andfhigh impedance taps, coupling from saidl connection of the AGCcircuit across thetotal inductancc4 3g-discharge tube plate output circuit to said impedance eleofthe-:inductor 24 does not overload the video IF amplifier 14 lduring normal operation of the receiver. Experi-r mental results'show that practical applicationof thisnovel circuitV to conventional receivers easily vdoubles or trebles a given" fluctuation of f receiver `inputn signalV strength' Itis noted that the-diodes-ZS and V-li'cperated orr oppo-Y sitespolarityhalves of Vthe-video.carrier 64.. That' is'fto' say; tlievideo' signal demodulated 'by the vdiode 28': corre.-

snondstto the/negative going excursions of thejcarrier'l 7 Qiiwthefotli'er hand,` the sync-pulses peak detected.byfth'e'r diode -SGT-are obtained ifromf thepositive *going excursions ot=th`e 'carrier 64?l Itis, off'course; possible to-:revers'ethe' polarityA 'off either the. diode 50 Vor'the .diode.'28 .to ma'k'e.

bothitheIY AGC' system and the video'demodlation-work Undensucli.

fromiitliefsame'polarity of carrier excursion. conditions,- certain conventional modificationsiof.tlieljrest of-the'receiverwouldhave to be-made.

Itris further obvious'that the inducta'rice-24'altliugli- --beingshuntfedV through jth'ecapacitor 26 could befincoif- S0 m'ent.a*stdio"de connected in Yseries with aV iirst load circuiti-to form. an intelligence signalfdeniodulator, conneetionsplcingisaid signal ldemodulator'between the low impedancetapslofsaid .indnctance toY demodulate intellitelligence signal to theinput of said` signal ampliei', means for. developing va control voltagefor-said gain controlling means inl` accordance with the peak'carrier'excursions corresponding. to "saidpulse component,` said voltage de- 0"velopin`g means being responsiveonly to said pulse component'excurs'ions and'having input' terminals for receiving carrier-pulses 'andoutnut terminals for delivering control voltage,` connections-from said developing means input Vterminals to thejhigh impedance taps of said impedance 5 element'and connections from the output terminals of said voltage-developing `means'to the/input of gain controlling means'whereby variations in pulse-carrier amplitude are carriedy tocontrol the amplitude of signal delivered to said v-.first signal demodulator.

voltage developing meansincludes a-second diode connected .-infseries with a secondload circuit to form a peak detectoicircuitY ands-wherein there is additionally pro- ,.vided biasingmeans connected in serieswith said. second".Y

7' ldiodefor biasingzoi-.saidzdiodesulicently -to permit con- This frequencyis selected in..

-. carrier; ,anelectron discharge tube amplifier having atV 4. Apparatus according to claim 3 wherein said controlV duztion therethrough only during peaks of said modulation u ses.

p 5. In a radio frequency amplifier for modulated radio frequency carriers an electron discharge tube having a grid-cathode input circuit and an anode-cathode output circuit, an inductance element having high impedance takeoff and low impedance takeol, means for coupling said inductance element to the output circuit of said discharge tube, a rst diode connected in series with a rst load circuit to form a demodulator combination, connecv tions placing said demodulator combination across said inductance low impedance takeoff, a second diode connected in series with a second load circuit to form a peak detector combination, connections placing said peak detector combination across said inductance high impedance takeoff. biasing means in series with said second diode to bias said diode off to prevent conduction thereof for signals below a given amplitude, means for supplying signals to said discharge tube input circuit, and means responsive to the voltage across said second load circuit for adjusting the amplitude of signal applied to said discharge tube input circuit whereby to regulate the Voltage developed across said first load circuit.

6. Apparatus according to claim 5 wherein said rst and second diode are polarized to conduct current through said inductance in opposite directions from one another.

References Cited in the tile of this patent UNITED STATES PATENTS Number Name Date 2,252,066 Dallos Aug. l2, 1941 2,254,855 Poeh Sept. 2, 1941 2,259,520 Freeman Oct. 2l, 1941 2,402,096 Somers .lune 11, 1946 2,498,839 Hayward Feb. 28, 1950 OTHER REFERENCES Electronics November 1938, page 16 (from series of articles on Practical Design and Performance Considerations on Television Receivers by Engstrom and Holmes).

Riders Television Manual, vol. 4, November 25, 1949, pages cited: Sears Roebuck TV, pages 4-6; Starrett TV, pages 4 6; Westinghouse TV, pages 4-6. 

